Sandstones, pebbly sandstones and conglomerates can be distinguished from
all other rock types by their grainy appearance. Individual grains are visible
to the naked eye and, in most cases, the rocks are very gritty to the touch.
Grains in sandstones range upward to about two millimeters in diameter (about
the diameter of the "lead" in a common pencil) and pebble size in pebbly
sandstones and conglomerates ranges up to several inches.

Sandstones

The basic criterion for distinguishing between different kinds of sandstone
is the character of the grains. It is difficult, in most cases, to determine
the attributes of individual grains, but the properties of the grains influence
the color and hardness of the rock and these two characteristics are readily
observable. Based on hardness and color, four main types of sandstone can
be recognized: (1) gray sandstone, (2) crystallized sandstone, (3) hard sandstone
and (4) carbonate cemented sandstone.

Gray Sandstones are a mixture of dark and light grains yielding a
rock that is gray in color and usually not as hard as other kinds of sandstone.
Hardness may be determined by breaking the core and by examining the edge
of the broken core. The edge of gray sandstone core is relatively weak and
crumbles either to the touch or with minor impact. Cores of other sandstone
types fracture with sharp edges or shatter into small sharp fragments.

Crystallized Sandstones are white and sugary and, in many cases, very
hard and brittle. In some examples of this type, the grains are welded together
and the cored rock has a smooth, polished appearance
(e.g. 23R).

Hard Sandstones have characteristics intermediate between gray sandstones
and crystallized sandstones, being light to dark gray in color like gray
sandstones, but hard and brittle like crystallized sandstones. Some examples
of hard sandstones are specimens
24L,
24R, and
upper 28R (this is the broken
butt end of a hard sandstone core). This butt end should be compared to
lower 28L, which is a mixture
of light gray, dark gray and tan grains. It should also be compared to the
pure white grains in the crystallized sandstone specimen
(e.g. upper 28L).

Carbonate Cemented Sandstones are relatively rare and usually are
gray sandstones in which the grains are held together with either calcium
or iron carbonate cement. Cements of this kind make the core hard and often
produce a brown or yellow stain on the sides of the core
(e.g. 26R). Calcium carbonate
cement can be detected by effervescence (bubbling) when 10% hydrochloric
acid is applied to the rock. Iron carbonates do not effervesce readily but
have a deep brown, sugary appearance on the broken butt end of the core
(e.g. lower 28R).

The second property for differentiating among sandstones is beddinga
property which is visible on the side of the core. There are five
typesmassive, crossbedded, streaked with shale, rooted and burrowed.
Examples of massive bedding in gray, crystallized, hard and carbonate cemented
sandstone are specimens 19L,
22L,
25L, and
26L, respectively.

The most common bedding type in sandstone consists of thin streaks or bands
of shale. If the streaks are inclined to the side of the core, the rock is
"crossbedded" (e.g. 29L), but
if the streaks are at right angles to the side of the core or nearly so,
the rock is designated as a sandstone with shale streaks. Specimen
31L is an example of a Gray
Sandstone with Shale Streaks and
34L is an example of a
Crystallized Sandstone with Shale Streaks. In some cases it is desirable
to describe different kinds of shale streaks. The most common type is "rippled",
(e.g. 31R) and a less common
type is designated "flat" (e.g.
33R).

Rooted and burrowed bedding are less common than other bedding types and
both are the result of organic activity on the sediment roots record plant
activity and burrows result from the action of burrowing invertebrate organisms
(e.g. clams and worms). Both produce a spotted or mottled appearance to the
rock (e.g. 36L and
38L), but roots can be distinguished
from burrows by the presence of thin films of coaly matter which represent
remnants of the plants and which are not present in animal burrows.

Pebbly Sandstones and Conglomerates

Criteria for identifying different kinds of pebbly sandstones and conglomerates
are the character of the sand between the pebbles and the kinds of pebbles
themselves. The character of the sand between the pebbles is the same as
the different kinds of sandstones1) gray (2) crystallized, (3) hard
and (4) carbonate cementedbut gray sandstone is the most common. The
most common pebble types include coal spars, coal bands (which are made up
of smaller coal fragments occurring as distinct layers), shale pebbles, ironstone
pebbles, quartz pebbles and rock pebbles. Coal spars
(e.g. 39L) are shiny, irregularly
shaped pieces of bright coal, whereas coal bands occur as distinct layers
(e.g. 39R). Shale pebbles
are usually dark gray or black (but not shiny like coal) with frayed or worn
edges like specimen 40L. Ironstone
pebbles, illustrated on 40R,
are usually oval or round and have a distinct orange-brown color. Quartz
pebbles are white or light gray and are often best observed when the core
is wet like those in specimen
43R. Rock pebbles are usually
harder than shale pebbles (compare
41R and
40L) and lack frayed boundaries.
Rock pebbles are rare but, when present, may include granite, gneiss, phyllite,
slate, chert and volcanics. Pebble types can occur in all possible combinations
but usually are found in one of the following groups:

Coal spars
Coal bands, with or without some coal spars
Shale pebbles, with or without some coal and/or quartz pebbles
Ironstone pebbles, with or without some coal and/or quartz pebbles
Shale and ironstone pebbles, with or without some coal and/or quartz
pebbles
Rock pebbles, with or without some coal and/or quartz pebbles
Quartz pebbles, with or without some coal

A key for identifying the commonly occurring types of sandstones, pebbly
sandstones and conglomerates is given in Table 2.